Hydraulically-actuated fuel injectors typically use a high pressure fluid acting on a relatively large area intensifier piston to compress fuel under a smaller area plunger. When fuel pressure is raised above a valve opening pressure, a needle check valve lifts to open the nozzle outlet, and fuel commences to spray into the combustion space within an engine. Although fuel could be used as both the hydraulic medium and injection medium, Caterpillar, Inc. of Peoria, Ill. has encountered considerable success by using high pressure engine lubricating oil as the hydraulic medium in its hydraulically-actuated fuel injection systems.
In order to accurately control the timing of each injection event, these fuel injectors typically include a solenoid actuated control valve that opens and closes the fuel injector to a source of high pressure actuation fluid, such as a common rail containing pressurized lubricating oil. Each injection event is initiated by energizing the solenoid to move the control valve to an open position, and each injection event is ended by moving the control valve back to its closed position.
Although these electronically-controlled hydraulically-actuated fuel injectors have de-coupled the injection amount and timing from the operation of the engine, there remains room for improvement, particularly in decreasing noise, particulates and NOx emissions from an engine. In this regard, engineers have observed that undesirable emissions over a significant range of an engine's operation can be decreased if each injection event is rate shaped to include a relatively small pilot flow rate at the beginning of an injection event followed by a relatively large flow rate in the main injection portion.
The present invention is directed to these and other problems associated with producing a particular rate shape trace in a hydraulically-actuated fuel injector.